The present invention relates to antenna systems and, more particularly, to devices for mechanically changing the polarisation of such antenna systems.
The revolution in telecommunications in the past decade has led to numerous developments in all fields of communications and data processing. It has also led to a corresponding increase in not only data traffic but also in the need for individuals to be constantly in communication with their colleagues. Such a need has been so pervasive that even while individuals are in transit, such as when travelling by air, data communications with their office computers, staff, and colleagues is vital.
To meet the above need for such communications, onboard data communications systems for aircraft have been developed. Such systems include antenna subsystems that track and communicate with satellites that relay data signals from the aircraft to the ground. Such data signals are ideally transmitted to the satellites with as little loss as possible to maintain the integrity of the signal. One source of signal degradation is polarisation loss. When an antenna receives linear polarisation from a satellite, the local polarisation in the coordinates of the antenna is dependent on the location of the antenna relative to the satellite as well as the orientation of the antenna relative to the satellite. If the antenna is mounted on an aircraft, then the position and orientation of the antenna will vary as the aircraft moves. This motion requires that the polarisation of the antenna also varies with time to ensure that polarisation loss is minimized.
While polarisation synthesis techniques may be used to compensate for the polarisation loss, in order to use such techniques it is necessary to have two orthogonal polarisation components excited in the antenna feed at each frequency of interest. This precludes the implementation of systems where the feed has only inputs that correspond to a single polarisation in each ban of interest.
Other polarisation synthesis techniques require more complexity and, hence, added cost. These complex implementations use two polarisation components in each band of interest. It is noted that some systems may have a single broadband feed channel in each of the two polarisation components. However, for full-duplex operation, each of the channels would have to be split into transmit and receive paths after the feed, resulting in a complex system having four polarisation/frequency ports.
Another possible solution to the polarisation loss problem is the use of ferrite devices for Faraday rotation of the electrical fields. Ferrite devices are attractive for polarisation control where they can be used. Unfortunately, such devices suffer from narrow bandwidth and high loss, making them unsuitable for simultaneous operation at 12 GHz and 14 GHz and also resulting in high noise temperature and reduced EIRP (Effective Isotropic Radiated Power) for a given input power.
Based on the above, there is a need for a low-cost and simple solution for compensating for polarisation loss.
The present invention seeks to overcome the above problems by providing systems and devices for rotating the polarisation of a signal emanating from or being received by an antenna system through mechanical means. The rotation of the polarisation is achieved, as in the prior art, by mechanically rotating the feed receiving or transmitting the signal. A non-metallic drive cord or belt is used to transfer motion from a motor located outside or behind the aperture to the feed polarisation axis. For a linear array of multiple antenna elements, each feed for each antenna element is rotated simultaneously and by an equal amount through the use of a drive system common to all the feeds. The drive system is coupled to each feed and to a drive motor. When the motor is activated, the drive system simultaneously rotates each feed by a given amount. By rotating the feed, the polarisation of the signal is correspondingly rotated and compensation for polarisation loss is provided.
In one aspect the present invention provides, an antenna element including:
a reflective element having a reflecting surface;
a feed rotatable about an axis;
rotating means for rotating the feed about the axis, the rotating means being coupled to the feed,
xe2x80x83wherein the reflecting surface faces the feed and rotation of the feed changes a polarisation signal emanating from the antenna element or being received by the antenna element.
In a second aspect the present invention provides an antenna element including:
a reflective element having a reflecting surface;
a non-metallic drive cord or belt;
a feed rotatable about an axis;
a drive motor located outside or behind the radiating aperture;
xe2x80x83wherein the reflecting surface faces the feed and rotation of the feed changes a polarisation signal emanating from the antenna element or being received by the antenna element, and rotation of the motor shaft moves the drive cord or belt causing the feed to rotate.
In a third aspect the present invention provides an array of at least two antenna elements, each antenna element including:
a reflective element having a reflecting surface;
a feed rotatable about an axis; the array including a common rotating means for rotating each feed of each antenna element, the common rotating means being coupled to each feed,
xe2x80x83wherein each reflecting surface faces a corresponding feed and activation of the common rotating means rotates each feed simultaneously.
an antenna element including:
a reflective element having a reflecting surface;
a feed rotatable about an axis;
a non-metallic cord or belt for rotating the feed about the axis, the rotating means being coupled to the feed,
a drive motor located outside or behind the radiating aperture and connected to the cord or belt,
xe2x80x83wherein the reflecting surface faces the feed and rotation of the feed changes a polarisation signal emanating from the antenna element or being received by the antenna element.